This invention relates to optical devices and, more particularly, to a technique for modifying one or more transmission characteristics of such devices.
Optical devices such as waveguides, gratings and switches, for example, are typically fabricated in layers of silica-based glass deposited onto a silicon substrate. The finished devices are expensive because an extremely high-precision fabrication process is involved. Moreover, even when the fabrication process is perfect, certain problems such as strain birefingence arise because the various layers have different thermal expansion coefficients, which cause large strains once the device returns to ambient temperature after undergoing annealing. It is therefore desirable to further process a fabricated optical device in order to modify one or more of its transmission characteristics (e.g., birefringence, phase shift and loss) to thereby improve yield and provide customization. is known to reduce birefingence by applying various forms of electromagnetic radiation such as ultraviolet (UV), x-ray, and even ionized particles such as proton beams. For example, Hibino et al., Electronics Letters, Vol. 29, No. 7 pp. 621-623 (1993) indicate that birefringence can be reduced by irradiating the waveguide structures at a wavelength which is absorbed by photosensitive defects in the core material. In particular, Hibino demonstrated that birefringence can be reduced in germanium-doped planar silica waveguides by irradiation with ultraviolet light at a wavelength of 248 nanometers (nm). This wavelength corresponds to germanium-related color center defects that are known to be present in the doped silica core material. In Hibino, the photosensitivity of the germanium core is enhanced by consolidating the core material in a reducing atmosphere. This technique is well known to facilitate the generation of germanium defects responsible for optical absorption at 248 nm. Since the cladding composition is presumably a standard phosphorous and boron-doped silica layer, the cladding is essentially transparent to light at 248 nm, with an optical absorption coefficient on the order of 0.1 cmxe2x88x921. By comparison, the optical absorption coefficient of the germanium-doped core layer is on the order of 100 cmxe2x88x921 at this wavelength. While the prior art has reduced birefringence in waveguide structures using ultraviolet (UV) radiation, these techniques have required that a specific mask be fabricated for each device and are unsatisfactory when applied to an integrated optical grating where 10-200 waveguides are involved, each separated by as little as 20 microns. In this case the waveguides cannot be selectively irradiated by masking individual ones of the waveguides. Moreover, the time required for birefringence modification is in the order of hours, which is too long for commercial application.
It is also known to cause refractive index changes by laser irradiation. Such changes may be used to modify the propagation delay (i.e., phase shift) of an optical waveguide in order to correct phase error. For example, Hibino et al., IEEE Photonics Technology Letters, Vol. 3, No. 7 pp. 640-642 (1991) apply an Ar+ laser, operating at 480 nm, for about one hour to modify the phase characteristics of a Mach-Zehnder interferometer via two-photon absorption. In a manufacturing environment, however, it is not practical to expend this much time per device. The photosensitivity of silica materials is sometimes employed to alter the refractive index without affecting the birefringence. For example, Kitagawa et al., Electronics Letters, 1994, Vol. 30, No. 16 pp. 1311-1312, form optical gratings by photo-inducing (via a mask) spatially non-uniform refractive index changes at a wavelength of 193 nm. The waveguides are first loaded with molecular hydrogen to increase their photosensitivity. Other techniques for modifying the phase shift of an optical waveguide involve: (i) the use of thin-film heaters deposited on top of the waveguides to thermo-optically change the propagation constant of the waveguide (H. Yamada, Electronics Letters, Vol. 31, No. 5, 1995, pp. 360-361); and (ii) the use of a thin amorphous silicon film deposited on top of the waveguides to change the propagation constant of the waveguide (H. Yamada, Electronics Letters, Vol. 32, No. 17, 1996, pp. 1580-1582).
Additionally, it is desirable to equalize the transmission loss associated with the input and/or output ports of a optical branching device during fabrication or by post-fabrication processing rather than by the addition of discrete attenuator elements. For example, a dense wavelength-division multiplexer (DWDM) is shown in application Ser. No. 08/759,281 using thermo-optic Mach-Zehnder interferometers as tunable attenuators. Equalization is difficult to achieve in a manufacturing environment, but it is critical to the proper operation of DWDMs and similar optical devices.
Accordingly, what is desired is a method for modifying one or more transmission characteristics of an optical device during fabrication or by post-fabrication processing rather than the addition of additional elements. Moreover it is desirable that overall processing be completed in less time than existing techniques permit.
The inventors of the present invention have discovered that by applying localized thermal treatment, of suitable intensity, that various transmission characteristics of an optical device can be modified. The optical device is fabricated from multiple layers of silica-based glass deposited onto a silicon substrate.
In an illustrative embodiment of the present invention, a carbon-dioxide (CO2) laser is used to selectively soften the cladding material of a waveguide structure. Such softening relieves the strain developed between the waveguide structure and the silicon substrate and substantially reduces or eliminates birefringence.
In another illustrative embodiment of the invention, the C2 laser is operated at a power level, which is suitable for changing the index of refraction the waveguide. Such modification changes the speed of light through the waveguide and, consequently, the phase shift associated with a given length of the waveguide is changed.
In yet another illustrative embodiment of the invention, the CO2 laser is operated at a power level, which is suitable for increasing the transmission loss of the waveguide. This is a particularly useful application of the invention because it allows the manufacturer to equalize the power levels of the various outputs of an optical demultiplexer, for example. Such equalization is frequently handled by attaching discrete optical attenuators to each output portxe2x80x94a technique that is costly, cumbersome, and prone to error.
In all of the embodiments, one or more transmission characteristics of an optical device may be modified by localized thermal treatment during, or subsequent to, fabrication of the device. Such treatment is substantially faster and/or less expensive than known techniques for achieving similar results, and are particularly advantageous for correcting imperfections associated with the manufacturing process.